1. Field of the Invention
This invention relates to a system for displaying the residual capacity of a secondary battery widely used as a power source for portable electrical apparatuses and also for generating an alarm signal when the residual capacity of the battery becomes smaller than a predetermined setting.
2. Description of the Prior Art
Prior art systems for displaying the residual capacity of a battery are of a battery voltage detection type which is advantageous from the aspect of cost, an electrical quantity integration type which ensures a highly precise display, etc. Such a system is disclosed in, for example, U.S. Pat. No. 4,377,787 and is mostly built in a portable electrical apparatus or a battery pack.
A prior art residual capacity displaying system of the electrical quantity integration type will now be described by reference to the accompanying drawings.
FIG. 7 shows the structure of such a prior art residual capacity displaying system. Referring to FIG. 7, the reference numeral 1 designates a secondary battery, and a current detection resistor 2 is connected at one end to the negative terminal of the secondary battery 1. A charging and discharge current detection section 3 is composed of a charging current detection operational amplifier 31 and a discharge current detection operational amplifier 32 to each of which a very small voltage appearing across the current detection resistor 2 is applied as its input. A voltage drop detection section 4 receives the terminal voltage of the secondary battery 1 as its input. A temperature detection section 5 detects the ambient temperature of the secondary battery 1. An electrical quantity computation section 6 receives the output signals of the operational amplifiers 31, 32 and the output signal of the temperature detection section 5 as its inputs. A charging completion detection section 7 receives the output signal of the operational amplifier 31 as its input. A capacity memory section 9 receives, as its input, an output signal of an electrical quantity integration section 10 which will be described now. This electrical quantity integration section 10 receives the output signal of the electrical quantity computation section 6, the output signal of the capacity memory section 9, the output signal of the voltage drop detection section 4 and the output signal of the charging completion detection section 7 as its inputs. A display section 11 receives the output signal of the electrical quantity integration section 10 as its input. A one-chip microcomputer 12 constitutes the circuitry of the electrical quantity computation section 6, charging completion detection section 7, capacity memory section 9 and electrical quantity integration section 10. The reference numeral 13 designates a terminal across which a portable electrical apparatus or a battery charger is connected.
The operation of the prior art residual capacity displaying system having the aforementioned structure will now be described.
The operation of the residual capacity displaying system in the charging mode will be first described by reference to FIG. 7. When the charger is connected across the terminal 13 to start charging the secondary battery 1, a negative very small voltage (when the level of the negative electrode of the secondary battery 1 is supposed to be set at a zero potential level) proportional to the charging current appears across the current detection resistor 2. This very small voltage is amplified up to a positive value by the function of the operational amplifier 31, and such an output signal of the operational amplifier 31 is applied to both the electrical quantity computation section 6 and the charging completion detection section 7. The electrical quantity computation section 6 computes the quantity of electricity by multiplying the charging current value outputted from the operational amplifier 31 by a predetermined period. Further, in the electrical quantity computation section 6, the computed quantity of electricity is then multiplied by the charging efficiency determined by both the level of the charging current and the temperature information supplied from the temperature detection section 5, thereby computing the finally charged quantity of electricity and applying such an output signal as an input to the electrical quantity integration section 10. In response to the application of the charging current signal from the operational amplifier 31, the charging completion detection section 7 resets its output. Then, when the charging current value changes from that of quick charging to that of trickle charging as the charging operation proceeds, the charging completion detection section 7 recognizes the end of the charging operation and sets its output. In the electrical quantity integration section 10, the charged electrical quantity signals successively outputted from the electrical quantity computation section 6 are added at time intervals of the predetermined period thereby integrating the quantity of electricity. Thus, the ratio of the integrated value of the quantity of electricity to the capacity value stored in the capacity memory section 9 determines the residual capacity of the secondary battery 1. When the charging completion detection section 7 sets its output, the integrated value of the quantity of electricity at that time is stored in the capacity memory section 9 to update the capacity at the time of the charging operation, so that the residual capacity of the secondary battery 1 is set at 100%.
On the basis of the residual capacity information determined by the electrical quantity integration section 10, the display section 11 displays the residual capacity stepwise on an LCD display as shown in FIG. 3A. As the secondary battery 1 is progressively charged, the residual capacity is successively displayed in the order of from a display level (1) towards a display level (6) as shown. FIG. 3A shows the relation between the residual capacity of the secondary battery 1 and the display level. The LCD display consists of five LCD segments horizontally arrayed on a line. In the display level (1) which indicates the residual capacity value of 0% to 10%, one LCD segment makes a flickering display. In the display level (2) which indicates the residual capacity value of 10% to 20%, one LCD segment is lit. Similarly, in the display levels (3) to (6) which indicate the residual capacity values of 20% to 40%, 40% to 60%, 60% to 80% and 80% to 100% respectively, two LCD segments, three LCD segments, four LCD segments and five LCD segments are lit respectively as shown. Thus, the residual capacity of the secondary battery 1 is continuously displayed by the lighting of one or more of the linearly arrayed LCD segments.
The operation of the residual capacity displaying system in the discharge mode will next be described. When a portable electrical apparatus to be used is connected across the terminal 13, the current is supplied from the secondary battery 1, and the discharge is started. As a result, a positive very small voltage (when the level of the negative electrode of the secondary battery 1 is supposed to be set at a zero potential level) proportional to the discharge current appears across the current detection resistor 2. This very small voltage is amplified by the operational amplifier 32, and the amplifier output signal is applied as an input to the electrical quantity computation section 6. The electrical quantity computation section 6 computes the quantity of electricity by multiplying the discharge current value outputted from the operational amplifier 32 by a predetermined period. Further, in the electrical quantity computation section 6, the computed quantity of electricity is multiplied by the discharge efficiency determined by both the level of the discharge current and the temperature information supplied from the temperature detection section 5, thereby computing the finally discharged quantity of electricity and applying such an output signal as an input to the electrical quantity integration section 10. The voltage drop detection section 4, which is basically in the form of a voltage comparator, continuously checks the secondary battery 1 for the voltage drop occurring as the discharge proceeds, and the level of the detection voltage is selected to be higher than the operation halting voltage level of the portable electrical apparatus. In the electrical quantity integration section 10, the discharged electrical quantity signals successively outputted from the electrical quantity computation section 6 are subtracted at time intervals of the predetermined period thereby integrating the quantity of electricity. On the basis of the ratio of this integrated value of the quantity of electricity to the value stored in the capacity memory section 9, the value of the residual capacity is determined. When the voltage drop signal is generated from the voltage drop detection section 4, the value stored in the capacity memory section 9 is corrected on the basis of the difference between the integrated value of the quantity of electricity at that time and the zero integrated value, thereby updating the residual capacity in the discharge mode. Then, this integrated value of the quantity of electricity is nullified to set the residual capacity value at 0%. More concretely, when the integrated value of the quantity of electricity at the time of appearance of the voltage drop signal from the voltage drop detection section 4 corresponds to 5% in terms of the residual capacity value, the manner of correction is such that the value stored in the capacity memory section 9 is decreased by the amount of 5%. On the other hand, when the integrated value of the quantity of electricity at the time of appearance of the voltage drop signal is negative due to subtraction and corresponds to -5% in terms of the residual capacity value, the manner of correction is such that the value stored in the capacity memory section 9 is increased by the amount of 5%. The display section 11 displays stepwise the residual capacity on the basis of the residual capacity information determined by the electrical quantity integration section 10, and its display successively shifts from the display level (6) towards the display level (1) shown in FIG. 3A as the discharge proceeds.
The operation of the residual capacity displaying system in the stand-by mode will be finally described. The stand-by mode designates the state where neither the portable electrical apparatus nor the battery charger is connected across the terminal 13 or the state where neither the charging current nor the discharge current is flowing even when the portable electrical apparatus or the charger is actually connected across the terminal 13. Therefore, the stand-by mode is recognized when the electrical quantity computation section 6 detects that both the output signal of the operational amplifier 31 and that of the operational amplifier 32 are null.
The electrical quantity computation section 6 determines the quantity of electricity discharged in the stand-by mode on the basis of the self-discharge quantity of electricity from the secondary battery 1 measured beforehand for each of individual ambient temperatures, the information regarding the electrical quantity consumption of the residual capacity displaying system for the secondary battery 1 and the temperature information supplied by the output signal of the temperature detection section 5. The later operation is the same as that in the discharge mode described already, so that any detailed description will be unnecessary.
It will be seen from the above description that, in the prior art battery residual capacity displaying system of the electrical quantity integration type, the quantity of electricity in the charging mode is corrected on the basis of both the level of the charging current and the detected ambient temperature, while the quantity of electricity in the discharge mode (including the stand-by mode) is corrected on the basis of both the level of the discharge current and the detected ambient temperature. The corrected quantity of electricity is integrated by addition in the charging mode, while the corrected quantity of electricity is integrated by subtraction in the discharge mode, and the integrated value is then compared with the stored value that is the newest capacity value, thereby determining the residual capacity level to be displayed.
Accordingly, in order to improve the precision of display, updating of the stored capacity value so as always to display its accurate value is important. In the prior art system, the stored capacity value is updated when the completion of charging is detected or when the voltage drop is detected. However, when the charging and discharge are repeated several times until the completion of charging is detected or the voltage drop is detected, an error between the integrated value of the quantity of electricity (the displayed value) and the actual value of the residual capacity tends to be increased due to possible errors attributable to the pre-set charging and discharge efficiencies. Thus, the prior art system has had the problem that updating of the stored residual capacity value under such a situation cannot always achieve the desired accurate updating of the residual capacity value, and the precision of display cannot be improved. SUMMARY OF THE INVENTION
With a view to solve the prior art problem described above, it is an object of the present invention to provide a battery residual capacity displaying system in which the stored residual capacity value can be accurately updated.
Another object of the present invention is to provide a battery residual capacity displaying system which can display the residual capacity of the battery with a high precision.
In accordance with the present invention which attains the above objects, there is provided a system for displaying the residual capacity of a secondary battery, comprising:
a current detection resistor for converting a charging current and a discharge current of the secondary battery into very small voltages respectively;
a charging and discharge current detection section composed of a charging side operational amplifier and a discharge side operational amplifier for amplifying the very small voltages appearing across the current detection resistor respectively and outputting the resultant signals respectively;
a voltage drop detection section for detecting the situation where the voltage of the secondary battery has dropped to a level lower than a predetermined setting;
a temperature detection section for detecting the ambient temperature of the secondary battery and outputting the resultant signal;
an electrical quantity computation section receiving the charging current detection output signal and the discharge current detection output signal of the charging and discharge current detection section together with the output signal of the temperature detection section as its inputs for deciding whether the operation mode is the charging mode, the discharge mode or the stand-by mode and computing the quantity of electricity in that mode;
a charging completion detection section setting its output signal when completion of charging the secondary battery is detected, but resetting its output signal when charging the secondary battery is started;
a discharged electrical quantity integration section for integrating the discharged quantity of electricity at time intervals of a predetermined period when the output signal of the charging completion detection section is in the set state and the output signal of the electrical quantity computation section indicates the discharge mode, but, when the output signal of the charging completion detection section is in the reset state, nullifying the integrated value irrespective of the output signal of the electrical quantity computation section;
a capacity memory section storing the capacity value integrated by the discharged electrical quantity integration section as a newest battery capacity when the output signal of the charging completion detection section is in the set state and the voltage drop signal is first outputted from the voltage drop detection section after the charging is started;
an electrical quantity integration section for integrating, at time intervals of the predetermined period, the quantity of electricity by addition when the output signal of the electrical quantity computation section represents the electrical quantity in the charging mode, but by subtraction when the output signal of the electrical quantity computation section represents the electrical quantity in the discharge mode, thereby determining the value of the residual capacity on the basis of the ratio of the integrated value of the quantity of electricity to the capacity value stored in the capacity memory section, the electrical quantity integration section setting the integrated value of the quantity of electricity to correspond to the value of the residual capacity at the predetermined voltage level when the voltage drop signal is first outputted from the voltage drop detection section after the charging is started, but setting the integrated value of the quantity of electricity to correspond to the residual capacity of 100% when the output signal of the charging completion detection section is in the set state as a result of the charging; and
a display section for displaying the residual capacity stepwise on the basis of the value of the residual capacity determined by the electrical quantity integration section.
The electrical quantity computation section decides whether the operation mode is the charging mode, the discharge mode or the stand-by mode. When the operation mode is decided to be the charging mode or the discharge mode, the charging current output value or the discharge current output value of the charging and discharge current detection section is multiplied by a predetermined period to compute the quantity of electricity, and the computed quantity of electricity is then multiplied by the charging efficiency or the discharge efficiency determined on the basis of both the charging current output value or the discharge current output value and the ambient temperature of the battery to compute the final quantity of electricity. On the other hand, when the operation mode is decided to be the stand-by mode, it is preferable to compute the quantity of electricity by selecting suitable temperature data meeting the detected temperature from among those data prepared beforehand for each of the individual ambient temperatures and representing both the self-discharge quantity of electricity of the secondary battery and the quantity of electricity consumed by the battery residual capacity displaying system.
Such an arrangement is advantageous in that the residual capacity of the battery can be updated to an accurate newest capacity value by integrating the quantity of electricity discharged from the battery from its completely charged condition to the time of first detection of the voltage drop signal without recharging, and by storing this integrated value in the capacity memory section as the newest capacity value.
A modification of the residual capacity displaying system according to the present invention further comprises a residual capacity alarm section which receives the terminal voltage of the secondary battery and the discharge current output signal of the charging and discharge current detection section as its inputs and correcting to lower the battery voltage detection level in proportion to an increase in the discharge current, thereby generating an alarm signal irrespective of the value of the discharge current when a predetermined setting of the residual capacity of the secondary battery is reached. When the alarm signal is first generated from the residual capacity alarm section after the battery is charged, the capacity memory section stores a value obtained by dividing the value of the quantity of electricity integrated by the discharged electrical quantity integration section by a factor determined by a predetermined residual capacity alarm level of the residual capacity alarm section (which factor is 1-0.1 =0.9 when the predetermined residual capacity level is 10%), and the resultant value is stored as a newest battery capacity in the capacity memory section.
When the battery voltage drop detection level in the residual capacity alarm section is set at a value (a residual capacity of about 10% to 30%) sufficiently higher than an operation halting voltage of a portable electrical apparatus, the frequency of updating the stored capacity value under the actually operating condition of the portable electrical apparatus can be increased. However, even when the battery voltage drop detection level is merely raised, the desired accurate capacity value cannot be stored in the capacity memory section due to the fact that the residual capacity at the time of battery voltage drop detection differs depending on the level of the discharge current. This is because the value of the residual capacity at the time of battery voltage drop detection is large when the value of the discharge current is large, while the value of the residual capacity at the time of battery voltage drop detection is small when the value of the discharge current is small. Therefore, the correction to lower the battery voltage drop detection level in proportion to an increase in the value of the discharge current is made so as to generate the alarm signal when the value of the residual capacity reaches the predetermined level. In addition, the integrated value of the discharged quantity of electricity when the battery discharges the current from the time of completion of charging to the time of first detection of the alarm signal without being charged in the course of the discharge is divided by the factor determined on the basis of the value of the residual capacity at the time of detection of the alarm signal (which factor is, for example, 1-0.1=0.9 when the residual capacity value is 10%) to find the newest capacity value, so that the accurate newest capacity value can be updated with a high frequency.
Another modification of the residual capacity displaying system according to the present invention further comprises, in addition to the capacity memory section storing the newest battery capacity, a capacity rating memory section for storing the rated capacity of the secondary battery, a capacity comparison section receiving the capacity value stored in the capacity memory section and the capacity value stored in the rating memory section as its inputs to determine the degree of battery deterioration on the basis of the ratio of the capacity value stored in the capacity memory section to the capacity value stored in the rating memory section, and a display section displaying stepwise the degree of battery deterioration determined by the capacity comparison section.